In general, Cu—Cr sintered alloy is used in electrode material for vacuum circuit breakers. Such Cu—Cr sintered alloy is prepared in a manner: providing a powder mixture of Cu as a material having good conductivity and Cr as an arc-resisting component blended in a proper content ratio, compacting the powder mixture into a predetermined shape of compressed compact, and sintering the compressed compact in a non-oxidizing atmosphere such as vacuum. The Cu—Cr sintered alloy thus prepared is worked into an electrode.
The electrode material for vacuum circuit breakers of Cu—Cr sintered alloy of this fashion is known for its excellent suitability for the electrode use, because making Cr grain size therein fine for homogeneous metal texture can enhance the electrical properties of the alloy such as current interrupting performance and dielectric strength performance.
If Cr content is increased more than 40 percent by weight intending to prepare a Cu—Cr sintered alloy of high Cr content having good electrical properties, the sintered density of the alloy does not become high enough because of voids that will appear in the sintering treatment. Rolling the Cu—Cr sintered alloy intending to bring its density high, as a measure against this problem, is not good enough. Such treatment still has a disadvantage in that making the metal texture homogeneous is prevented by the aggregation of Cr caused from rolling.
If Cr powder of 10 μm or smaller in grain size is used in manufacturing Cu—Cr sintered alloy by an ordinary solid phase sintering process after mixing Cu powder and Cr powder together, oxidation process of Cr powder will occur making progress of sintering difficult with oxygen content increased. This invites poor electrical properties such as in the current interrupting performance and dielectric strength performance.
To overcome these problems, a material for electrical contact and manufacture thereof has been proposed in JP 04-95318 (Patent literature 1). In the art proposed in Patent literature 1, Cu—Cr sintered alloy uses atomized Cu—Cr alloy powder in which Cr of average particle diameter less than 5 μm is dispersed in a Cu-base metal (matrix). This atomized Cu—Cr alloy powder is prepared by the processes of: mixing 0.1 to 37 percent by weight of Cr powder with Cu powder, melting the powder mixture in inert gas atmosphere or vacuum, and solidifying the molten metal of the mixture with a rapid solidification method using an atomizer.
An atomized Cu—Cr alloy powder that includes 5 to 20 percent by weight of Cr is sintered intending to prepare an electrode material having improved electrical properties such as current interrupting performance, wherein the material contains homogeneously dispersed fine grain of Cr the average particle diameter of which in the Cu-base metal of the sintered compact is 2 to 20 μm.
As Patent literature 1 describes, the electrode material for vacuum circuit breakers made of Cu—Cr alloy powder prepared by atomizing process followed by solid phase sintering has an advantage of having good electrical properties. The Cu—Cr sintered alloy however has a problem in that manufacturing Cu—Cr sintered alloy of high Cr content is not practicable, because it is difficult to homogeneously disperse fine grain of Cr to a gross content of 30 percent by weight or more.
In atomizers usually used in mass production, it is their processing limit to manufacture Cr alloy powder containing 20 percent by weight of Cu. If the Cr content is increased more than that, the nozzle of such atomizer for spraying molten metal have a clogging problem.
There is another problem further to the clogging. If atomized Cu—Cr spherical powder is prepared with addition of Cu powder, which is a material having good press moldability and aggregability, intending to improve sinterability, the gross content of Cr in Cu—Cr sintered alloy will largely decrease with obtaining good electrical properties prevented.
An object of the present invention is to provide an electrode material for vacuum circuit breakers along with a method of manufacturing the same that is able to satisfy requirements by vacuum circuit breakers regarding low contact resistance value with improved electrical properties such as high-current interrupting performance and dielectric strength performance.